Surface wave launcher



Feb. 12, 1963 K. IKRATH 3,077,569

SURFACE WAVE LAUNCHER Filed Nov. 5, 1959 FIELD FICTiTIOUS MAG. DIPOLESINVENTOR, KURT IKRATH ...Wx/5. a.

ATTORNEY.

United States Patent @dice $77,559 Patented Feb. l2, 1963 3,677,569SURFACE WAVE LAUNCHER Kurt llirrath, 2*5 l-irhiand Ave., Elheron, Nil.,assigner to the United States of America as represented by the Secretaryoit the Army Filed Nov. 3, 1959, der. No. @563,742 5 Qlairns. (Cl. 33E-95) (Granted under Title 35, U.S. Code (1952), sec. Zoon The inventiondescribed herein may be manufactured and used by or for the Governmentfor governmental purposes without the payment of any royalty thereon.

This invention relates to a waveguide and more particularly to anelectromagnetic wave launcher which is capable of propagating surfacewaves along the boundaries of a current carrying wire mesh structure andmaterials of different electromagnetic properties. n

The electrical and mechanical features of the invention make it highlyuseful as a nonres-onant coupler of radio frequency power for feedingconventional surface wave transmission lines from conventional cablesand as a nonresonant travelling wave antenna element.

Wave launchers comprised of combinations of feeders, couplers, andantennas of various types have been used before in Iapplications t-owhich this invention is adopted but have proved to be unsatisfactory inmany respects. This invention provides a relatively'simple device toreplace the combinations of elements previously used.

It is well known that electromagnetic waves can be propagated alongsurfaces where discontinuities of the electromagnetic properties ofmedia exist; for example, along the plane or spherical boundariesbetween ground and air or water and air, along the cylindrical boundarysurface of copper wire and air and specially designed surfacetransmission lines utilizing dielectric coated wire and air as media(G-strings), along dielectric rods, and along cylindrical holes indielectric material. These surface wave modes of propagation prior toapplicants invention were excited by conventional methods with dipoles,probes, and open-ended waveguides such as horns and various combinationsthereof.

intimately linked with guided wave phenomena is radiation from theguided surface wave, i.e., the breaking away into space ofelectromagnetic energy from the guiding structures as a result ofdiscontinuities on the structure ofthe guide. Such a structure thenbecomes a travelling wave radiator and is known as a travelling wavewire antenna or dielectric antenna. These radiators have the commonproperty that they produce highly directional multi or single loberadiation patterns such that their directivity is proportional to theirlength, a distinctive characteristic of this type of antenna. Althoughthese antennas are inherently nonresonant wide band devices, in prac icebandwidth limitations are caused by the conventional feeder and couplersystems. 1

An object of this invention is to provide la unidirectional travellingwave antenna which physically incorporates feeder, coupler and radiatingaction into one element.

A second object of this invention is to provide a nonresonantdistributed coupling means -to a surface wave mode of electromagneticenergy from a conventional waveguide or transmission line mode of wavepropagation.

A further object of this invention is to provide a unidirectionaltravelling wave antenna whose physical length is less than that ofconventional long wire antennas with similar directionalcharacteristics.

A preferred form of the invention consists of a coaxial arrangement of acylindrical inner conductor electrically insulated from a cylindricalenvelope or enclosure which contains an inner layer in the forni or aconductive mesh of closely spaced apertures and an outer layer of highpermeability material of suitable thickness. The ends of the innerconductor and the inner layer of the envelope, in actual use of theinvention, are connected to the inner and outer conductors,respectively, of a cable connector to permit feeding from a conventionalcable and/or connection to the terminals of power generators and loads.

Another embodiment of the invention is a flat, sandwich-shaped variationof the above description.

Further objects and advantages of the invention will be apparent fromthe following description taken in connection with the drawing, wherein:

FGURES 1 and 2 illustrate in diagrammatic form the basic principles ofthe invention;

FIGURES 3 and 3a illustrate the invention in coaxial form;

FIGURES 4 and 4a illustrate a planar version of the invention;

FIGURE Str shows the mesh apertures of the outer conductors; and

FIGURE 5b graphically shows a single aperture for purpose of thelmathematical development.

The same reference numbers are used to represent the same or similarelements throughout the iigures Iof the drawing.

Referring to FIG. l of the drawing, a braided or meshwire conductor )itlof the type commonly used in coaxial transmission lines as the outerconducting element `has a primary magnetic field Hp induced in the spacebeneath it by the wave travelling through the line. Leakage of primaryiield Hp through the apertures li. into the originally ield-free spaceabove the apertures il at high frequencies exceeds by far the leakagethrough the metal part of braid l@ and is represented by tield lines i2.The field thus formed in the space above the apertures can be consideredto emanate from a fictitious magnetic and elec tric dipole, indicatedlschematically in the apertures 11. The close proximity of apertures liand hence the close spacing of the tictitious dipoles located within theapertures lll results in a strong coupling between the fictitiousdipoles to form the magnetic leakage r'icld ft2. The dipole conceptexplains the experimentally observed shape of the radiation pattern froma line constructed in this manner whose main lobes tend to emanate inthe direction of the exciting travelling wave and which have the forwardtilt of slow wave type radiation patterns. The length of the rneshwirelo has a direct effect on the sharpness of the lobes, i.e., the longerthe cylindrical wire mesh, the sharper is the directivity of the lobes.

ri`he mathematical development of this dipole concept will be describedin conjunction with FIGS. 5a and 5b which show portions of the outermeshwire conductor. FIG. 5b serves as an analytical model `forcomputation of the surface EMF. The magnetic field Hy above lsuchapertures (ie. the tield perpendicular to the plane of the apertureshown in the drawing) is represented by the equation:

Hy=Hy-l-Hy" (l) where Hy is the total held above the mesh, Hy is the eldcontributed by the current z" flowing in wires 25', and I y" is thefield contributed by the current i" in wires 35. Each individual fieldconiiguration can be represented in ter-ms of the dimensions of thebraid as shown in the following equation for Hy:

where z" is the current flowing in wires Z5 of the braided conductor, nis lthe number of meshes or apertures, a is the distance between theaxes of wires 25, and 5 is the aeration distance along the g axis of theoblique X, g, 7;, coordinate system from the origin to wire 25. Carryingout the summation of Equation where i" is the current ilowing in wires35 and i; is the distance along the axis of coordinate system X, 5, 17,from the origin to wire 35.

Adding Equations 3 and 4 according to Equation 1 the total magneticfield Hy in the plane of the mesh or where g is the distance between theoutside surfaces of adjacent parallel conductors in the braid.

The relationship shown in Equation is true for the following validityconditions:

(1) Stationary D.C. like behavior of the wire braid currents is assumed.

(2) Magnetic field distortions at the braid wire crossings areneglected.

(3) The portion of the braid under consideration is considered at andthe number of meshes or apertures in this portion is assumed to be large(ne oo).

Choosing the X and Z coordinates to be the diagonals of a diamond shapedaperture as shown in FIG. 5b, the magnitudes of E and v; can bemathematically expressed in terms of X, Z, and a so that X Z -2 cos a-2sin a (8) and X Z 712 cos et+-2 sin (9) The magnetic iield Hy found bymeans of Equation l() has been well confirmed experimentally therebyshowing the practical Validity of the mathematical approach which led tothe development of the practical, operative device to be described inconjunction with FIGURES 2, 3, `and 4. The magnetic iield Hy of Equationl0 is designated as magnetic leakage ield 12 in FIGURES l and 2.

According to Faradays law, a ring voltage is linked to the leakage iiuxdescribed above such that an electric eld is produced parallel to theresultant current ilow through the meshwire conductor. The mean electricfield strength in the plane of each mesh aperture is equivalent to anper unit length along the surface of the meshwire structure, This EME.per unit length is directly proportional to the primary magnetic fieldHp or what is equivalent to the current i in the meshwires, and itsphase Velocity equals the phase velecity of the primary field. The EMP.-thus produced excites and supports a surface wave on the surface of themeshwire conductor lo at high frequencies and is linked to the magneticleakage ux leaking through the apertures il so that the meshwire surfaceper unit length increases with increased 'frequency of the input wave.This increase ofthe surface EMF. per unit length causes a correspondingincrease of the ratio or" the radial to the longitudinal field energyiiow in the vicinity of the meshwire, t-h-at is, a trend from waveguidance to wave radiation of the slow wave type previously discussed.

According to the invention, if the meshwire 1t) is covered with a sheetof material 13, as shown in FIG. 2, having a high permeability constant(low reluctance), negligihle conductivity `and negligible hysteresisloss, the magnetic leakage iiux 12 through the apertures lll will tendto be confined within the highpermeability material 13 and will notstray into the outside space which has a higher reluctance. The densityof the magnetic iield l2 is greatly increased in cover sheet 13 as aresult. Referring again to the .fictitious dipole conception, it can besaid that the high permeability cover sheet 13 acts as a shunt or keeperfor the fictitious dipoles in much the same manneras a keeper for apermanent magnet. y

The meshwire conductor l@ and high permeability cover sheet 13 of properthickness, in effect act as a transformer to couple energy (e.g., a TEMmode) from below the meshwire conductor 1li to the region just above themeshwire conductor it) within cover sheet i3 where the energy ispropagated in the form of a surface wave. The high concentration ofmagnetic energy within cover sheet i3 will increase the per unit lengthalong lthe surface of meshwire conductor l0 to a value considerablyhigher than that obtained from the device of FIG, 1. The guided surfacewave thus formed is in a non-radiating mode tightly coupled to the wavebeneath meshwire conductor lit. The degree of this coupling andcharacter of the surface wave is controlled by the size and spacing ofapertures il and by the thickness and permeability of cover sheet I3.

Conversion of the non-radiating surface wave to a radiating mode canthen be accomplished by feeding the wave into a discontinuity at the endof a surface wave launcher of the type shown in FlG. 2. This will causea disruption of the intimate coupling between the surface wave and innerwave thereby resulting in the generation of a radiating mode which isreected back and forth between the ends of the launcher in a standingwave pattern being set up by the radiating modes such that there is anet energy flow from the input end to the load or terminal end of thewave launcher. This explains the directivity of the radiation patternand the proportionality of its sharpness with the length of thelauncher.

A preferred form of a surface wave launcher according to the inventionis shown in FIGS. 3 and 3a and consists of a coaxial arrangement of acylindrical inner conductor 14 surrounded by a dielectric material l5'having `a proper dielectric constant which insulates conductor M from anenvelope comprised or" an inner meshwire conductor i6 With a thin outercoating l? of a material having high permeability (ferrite forinstance). In actual use, a suitable source of high frequency power isconnected to one end of conductors 14 and 16 and the other end of thesurface wave launcher could be utilized for launching surface wavesalong a wire (Gf-string) or other suitable loads. `Construction `of thiscoaxial surface wave launcher in a conical shape will sharpen theradiation pattern considerably. Operation of the embodiment shown inIFIGS. 3 and 3a is otherwise the saine as described in conjunction withFIG. 2.

FIGS. 4 and 4a show a planar or sandwich version of the surface wavelauncher described in FIGS. 2, 3 and 3a. In this version a thin planarconductor 18 is sandwiched between two layers of insulating material 19each of which are in turn covered by a meshwire or braided conductor 20.The braided conductors 20 are then each covered with a sheet 21 ofmaterial having a high permeability. This form of the invention isparticularly useful for the excitation of surface waves along ice and`air or water and air boundaries.

The high permeability cover sheet may be made of ferrite and could beapplied to the coaxial version of the invention by winding ferrite tapearound the outer meshwire conductor of the cable, for example.

While the invention has been described with reference to particularembodiments, various other specific embodiments may suggest themselvesto those skilled in the art without departing from the spirit and scopeof the invention. Therefore, the present embodiments are to beconsidered illustrative only with reference being had to the appendedclaims to indicate the true scope of the invention.

What is claimed is:

1. A surface wave launcher comprising a rst conducting means, a secondconducting means of relatively open meshwire construction having twosurfaces, insulating means separating said first conducting means andone surface of said second conducting means, and a high magneticpermeability cover sheet substantially covering the other surface ofsaid second conducting means.

2. A surface wave launcher according to claim 1 wherein said insulationmeans has a dielectric constant which matches the phase velocity of theprimary eld within said insulation means to 4the surface wave phasevelocity.

3. An electromagnetic wave launcher comprising a pair of planarrelatively open meshwire conductors arranged in parallel spaced relationwith respect to one another, insulation means, a solid plane conductivesheet between said meshwire conductors and separated from each of saidmeshwire conductors by said insulation means, and a high magneticpermeability cover sheet substantially cov ering each of said meshwireconductors on the side remote from said insulation means.

4. A microwave antenna` comprising inner and outer conducting elementsarranged in coaxial relationship and separated by suitable insulatingmeans, said outer con ducting element having a plurality of closelyspaced apertures therein, and a. cylindrically shaped enclosure ofmaterial having a high magnetic permeability encasing said coaxialconducting elements and arranged in contact with said outer conductingelement.

5. The apparatus of claim 4 wherein said inner and outer conductingelements are adapted to be connected -to a suitable source of highfrequency power and where said insulating material has a dielectricconstant which matches the phase velocity of the primary eld within saidinsulating means to the phase velocity of a surface wave excited in saidhigh magnetic permeability enclosure.

References Cited in the file of this patent UNITED STATES PATENTS563,274 Guilleaume July 7, 1896 2,018,353 Gothe Oct. 22, 1935 2,111,651Wentz Mar. 22, 1938 2,894,226 Wild July 7, 1959 2,913,515 Ebel Nov. 17,1959 2,915,719 Larsen Dec. l, 1959 2,929,034 Doherty Mar. 15, 19602,938,943 Horn May 3l, 1960 2,994,050 Ayer July 25, 1961 FOREIGN PATENTS633,190 Great Britain Dec. 12, 1949 OTHER REFERENCES Rotman: A StudyGuides, published in Proceedings of the IRE., vol 39, Issue 8, pages952-959, August 1951.

Goubau: Surface Waves Lines, published in Journal of Applied Physics,November 1950, pages 111,94 1128, volv 2 1,No,11,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,077,569 February 12g 1963 Kurt Ikrath It s hereby Certified that ervrorappears n the above numbered patent requiring Correction and that thesaid Letters Patent should read as corrected below.

Column 3, lines 8 and 9 e uation (4) should a ea shown below lnstead ofas in'tlnaI patent: pp P as Signed and sealed this 22nd day of October1963 (SEAL) Attest:

EDWIN L., REYNOLDS ERNEST W. SWIDER ttesting Officer AC tingCommissioner of Patents UNITED STATES PATENT oEETCE CERTIFICATE OFCORRECTION Patent No. 3,077 ,569 February 12, 1963 Kurt Ikrath It ishereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 3, lin-es 8 and 9 e uation y(4l) should a ea shown below insteadof as in'tlneI patent: pp r as Signed and sealed this 22nd dey ofOctober 1963.

(SEAL) Attest:

EDWIN L REYNOLDS ERNEST W. SWIDER Attesting Officer Acting Commissionerof Patents

1. A SURFACE WAVE LAUNCHER COMPRISING A FIRST CONDUCTING MEANS, A SECONDCONDUCTING MEANS OF RELATIVELY OPEN MESHWIRE CONSTRUCTION HAVING TWOSURFACES, INSULATING MEANS SEPARATING SAID FIRST CONDUCTING MEANS ANDONE SURFACE OF SAID SECOND CONDUCTING MEANS, AND A HIGH MAGNETICPERMEABILITY COVER SHEET SUBSTANTIALLY COVERING THE OTHER SURFACE OFSAID SECOND CONDUCTING MEANS.